In a typical percutaneous transluminal coronary angioplasty (PTCA) procedure, a guiding catheter is percutaneously introduced into the cardiovascular system of a patient. The guide catheter is advanced through a vessel until the distal end thereof is at desired location in the vasculature. A guide wire and a dilatation catheter having a balloon on the distal end thereof are introduced into the guiding catheter with the guidewire sliding through the dilatation catheter. The guide wire is first advanced out of the guiding catheter into the patient's coronary vasculature, and the dilatation catheter is advanced over the previously advanced guide wire until the dilatation balloon is properly positioned across the lesion. Once in position, the flexible, expandable, preformed balloon is inflated to a predetermined size with a liquid or gas at relatively high pressures (e.g. about ten to twelve atmospheres) to radially compress the arthrosclerotic plaque in the lesion against the inside of the artery wall and thereby dilate the lumen of the artery. The balloon is then deflated to a small profile so that the dilatation catheter may be withdrawn from the patient's vasculature and blood flow resumed through the dilated artery.
In angioplasty procedures of the kind described above, there may occur a restenosis of the artery; i.e., a re-narrowing of the treated coronary artery which is related to the development of neo-intinmal hyperplasia that occurs within the artery after it has been treated as described above. In a sense, restenosis is scar tissue that forms in response to mechanical intervention within a vascular structure. To prevent restenosis and strengthen the area, an intravascular prosthesis generally referred to as a stent can be implanted for maintaining vascular patency inside the artery at the lesion. The stent is then expanded to a larger diameter for placement or implantation in the vasculature. This is often accomplished by the balloon portion of the catheter. The stent effectively overcomes the natural tendency of the vessel walls of some patients to close back down, thereby maintaining a normal flow of blood through the vessel that would not be possible if the stent was not in place.
A known expandable stent which is delivered on a balloon catheter may be considered to be a stainless steel cylinder having a number of openings in its circumference resulting in a scaffolding when expanded. The stainless steel cylinder is compressed onto the outside of a non-expanded balloon catheter which includes stent retainer rings at each end of the stent to help maintain the stent on the balloon. Unfortunately, the limited amount of securment between the stent and the balloon is not always adequate to insure that the stent will properly stay in place while advancing the stent to and through a target lesion. Additionally, the outer surface of the delivery device is uneven because the stent generally extends outwardly beyond the balloon. Thus, the stent may contact a narrow vessel wall and be displaced while the catheter negotiates a narrow vessel. Furthermore, during a coronary intervention, the physician may have difficulty crossing the target lesion. In such cases, it may be necessary to pull the stent delivery system back into the guide catheter. Such procedures can be risky because the stent may become caught on the edge of the guide catheter.
For example, the guide catheter is generally inserted through the abdominal aorta to a point just beyond the ostium, the location from which the right coronary artery and the left main artery diverge. Blockages or lesions are present in smaller coronary vessels, and medical practitioners may sometimes predilatate the target area as, for example, by balloon angioplasty. Sometimes, however, predilatation is not performed, and doctors proceed directly to a primary stenting procedure. In such cases, there are occasions when the balloon/stent catheter cannot be properly positioned within the target area due to the constriction of the vessel and must be retracted back into the guide catheter. Even when predilatation is performed, vascular spasms and/or a reclosure of the vessel may occur rendering it difficult to properly align the balloon/stent likewise requiring retraction into the guide catheter. It is during the retraction process that the stent can catch or impact the edge of the guide catheter causing it to be dislocated or otherwise damaged.
To migrate this problem, balloon pillow have been utilized to provide a smooth transition of the balloon/stent assembly back into the guide catheter. Such balloons may be produced through a process of heating and cooling in, for example, Teflon sheaths which are stacked in a way to produce a step in the collapsed balloon surface. One such system is shown and described in U.S. Pat. No. 5,836,965 issued Nov. 17, 1998 and entitled “Stent Delivery and Deployment Method.” In this apparatus, pillows formed in the balloon at opposite ends of the stent assist and secure the stent to the balloon and create a smooth transition between the stent area and the distal and proximal surface of the delivery device. The balloon may be tapered or non-tapered. Additionally, conventional retainers may be attached over the balloon or placed within the balloons. Unfortunately, the balloon material is generally very pliable, and it has been found that when using such devices, the pillows may collapse under the pressure of insertion and/or retraction thus exposing the edge of the stent to the potential impact on the edge of the guide catheter.
It should therefore be appreciated that it would be desirable to provide a low profile stent delivery apparatus which may be configured to have a generally smooth outer surface so as to avoid collisions between the stent and other obstructions such as the edge of the guide catheter.